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Installed mat and 2 pieces behind seats in my 2018 GS last weekend. Drove 180 miles thru Texas Hill Country on a variety of roads, asphalt old and new, concrete and I am very impressed with the noise being reduced by 1/2 according to my ears. I can notice the difference and the music is "clearer" at high speeds. After reading other posts I guess everybody's hearing is different. Just a FYI.
I added the BlockIt mats behind the seats then took two additional steps that greatly reduced the road noise even more. I lifted the carpet behind the seats and laid down Dynamat and then removed the rear wheel wells and laid down a double layer of Dynamat.
I also have used the Block It in my C6, when I had it and my current C7. I found that, like some of you here, the noise level was reduced. Not a difficult install either.
Installed mat and 2 pieces behind seats in my 2018 GS last weekend. Drove 180 miles thru Texas Hill Country on a variety of roads, asphalt old and new, concrete and I am very impressed with the noise being reduced by 1/2 according to my ears.
IMO the BlockIt mats are a good bang for the buck and easy mod to reduce annoying road noise...
I added both behind the seats and cargo mats. The cargo mat seemed to be the most helpful as the cargo compartment floor is essentially a drum head transmitting noise from the rear end upward to the rear window and reflecting it forward to the seats. In some non scientific "tests" using DecibelX Pro app on my iPhone showed a 3+ dB reduction in overall sound level driving the same sample routes
IMO the BlockIt mats are a good bang for the buck and easy mod to reduce annoying road noise...
I added both behind the seats and cargo mats. The cargo mat seemed to be the most helpful as the cargo compartment floor is essentially a drum head transmitting noise from the rear end upward to the rear window and reflecting it forward to the seats. In some non scientific "tests" using DecibelX Pro app on my iPhone showed a 3+ dB reduction in overall sound level driving the same sample routes
I too had added the BlockIt Cargo Mat, it too greatly decreases cabin road noise.
Blockit Mat installed
IMO the BlockIt mats are a good bang for the buck and easy mod to reduce annoying road noise... 
